Pat the tuna steak completely dry with paper towels and season all sides with sea salt and black pepper.
Place the sesame seeds on a small plate and press the tuna firmly into the seeds to create an even crust on all sides.
Heat the avocado oil in a cast-iron skillet over medium-high heat until it begins to shimmer.
Sear the tuna for 60-90 seconds per side for a rare center, then transfer to a cutting board to rest for 3 minutes.
In a small jar, whisk together the tamari, rice vinegar, toasted sesame oil, and freshly grated ginger to create the dressing.
Assemble the bowl by placing the cooked rice at the base and topping it with sliced cucumber, edamame, and avocado.
Slice the rested tuna into thin strips using a sharp knife and arrange them over the rice and vegetables.
Garnish with thinly sliced green onions and drizzle the prepared ginger-tamari sauce over the entire bowl.